Hydrogen production is commercially proven, but expensive. One method of producing hydrogen is steam methane reforming where hydrocarbons and water are reacted to form CO and H2, followed by a separate water-gas-shift reaction where CO is reacted with H2O to form CO2 and H2. The commercial application of these reactions in many refineries commonly involves a series of reactors including a steam reforming reactor, and several post reactors to address the production of CO in the reformer. The post reactors include a high temperature shift reactor, a low temperature shift reactor, and a CO2 absorber separator. Water and CO2 separation is necessary to achieve pure hydrogen. The reforming reactor is run at high pressure to avoid hydrogen recompression downstream. The pressure lowers the equilibrium conversion since reforming produces a positive net mole change. The steam reforming reaction is very endothermic, about 206 kJ/mole; and the shift reaction is exothermic, providing about 41 kJ/mole. The conventional steam reforming reactors are operated above 900° C. to push the equilibrium toward complete formation of CO and H2. The high temperature causes severe corrosion and stress problems on the equipment. Steam reforming reactors are generally large to accomplish economies of scale. Furthermore, designs currently known do not lend themselves to being scaled down to a smaller size or to making it possible to efficiently control the temperature at various points.
In production of hydrogen by conventional steam reforming processes like the one described above, only 50 percent to 60 percent of the heat generated in the process is used for hydrogen production. The remaining 40 percent to 50 percent is recovered for combustion air preheating, feed preheating, process steam generation, and export steam generation. The generation of export steam in a conventional steam reforming process is unavoidable due to the excess or waste heat leaving the steam reformer furnace in the form of hot flue gas.
In many refinery applications, there is no demand for the export steam that can be produced from the steam reformer. If the refinery cannot utilize the export steam, it is difficult for the conventional steam reforming process to efficiently utilize the energy consumed by the reformer and, thus, will face challenges to supply hydrogen on a cost-effective basis. Typical large steam reformers (80 to 100 mM SCF/D H2) typically run at a net efficiency of about 370 BTU/SCF H2 (with export steam) and a gross efficiency of about 410 BTU/SCF H2 (without export steam). The additional heat (30-40 BTU/SCF) that is captured and converted to export steam has a significant impact on the cost of hydrogen production. For example, waste heat captured and converted to export steam for a 100 mM SCF/D hydrogen plant the is worth approximately eight million dollars per year in energy costs. Thus, it would be desirable to capture this waste heat to improve the energy utilization and cost of hydrogen production of the conventional steam methane reformer.
Hydrogen can be produced in membrane steam reformer units. For example, U.S. Pat. No. 6,821,501 discloses an apparatus and method for steam reforming membrane reactor which includes membrane steam reformer tube comprising a membrane tube surrounded by a reaction tube forming an annulus that is packed with a reaction catalyst. The process allows for the combination of the reforming reaction CH4+H2OCO2+H2 and the shift reaction CO+H2OCO2+H2 to be combined in one reactor. Additionally, the process takes place at a temperature of from about 450° C. to about 550° C., significantly lower then conventional steam reforming units.
U.S. Pat. No. 5,861,137 discloses a compact, mobile steam reformer that includes a tubular hydrogen permeable and hydrogen selective membrane. A reforming bed surrounds at least part of the membrane. An inlet to the reforming bed receives a mixture of alcohol or hydrocarbon vapor and steam and an outlet from the reforming bed releases reforming byproduct gases. A heating element heats the reforming bed to an operating temperature and a second bed including a methanation catalyst is placed at the permeate side of the membrane. A reformer outlet withdraws hydrogen gas from the second bed. In one aspect, the heating element is a third bed including an oxidation catalyst surrounding at least a portion of the first bed. The reforming byproduct gases released from the reforming bed mix with an air source and catalytically ignite to generate heat and thermally support the process of reforming within the reforming bed.
Therefore it would be desirable in the art to provide a steam reformer reactor design that could utilize the waste heat of conventional heaters or steam methane reformer furnace. If the waste heat in the convection section of a heater or steam methane reformer furnace could be used to produce hydrogen, it would represent a distinct advance in the art. In addition, the if the process could be constructed for significantly lower capital costs and have lower operating costs, it would be desirable. Furthermore, if the process produced CO2 in higher concentrations and greater purity than other processes in the art, and the CO2 could be sequestered for other uses, it would be extremely desirable. Such an integrated system would demonstrate far greater efficiency than any hydrogen generating system currently available.